Total precast solution for large stadium projects meet tight schedule

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Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8 Total precast solution for large stadium projects meet tight schedule T.J.

Framing system design and construction methods are presented and details of the construction and design of three major league and college stadiums are presented.

1 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN Total precast solution for large stadium projects meet tight schedule T.J. D Arcy The Consulting Engineers Group, Inc., San Antonio, TX, USA ABSTRACT: A report on the successful application of precast prestressed concrete in large stadium projects. Framing system design and construction methods are presented and details of the construction and design of three major league and college stadiums are presented. Precast prestressed concrete has proven to be an excellent application in major stadium projects. The Consulting Engineers Group has provided engineering services for over 70 large stadium projects in North America. These applications include stadiums for National Football teams, Major League Baseball teams, National Basketball Association teams, Olympic Stadiums and various College Stadiums. Stadiums are designed in three manners; either they are open air, enclosed or combination stadiums. Even though they are very expensive, the current trend in the major league is to build enclosed stadiums with retractable roof structures. Thereby combining both open air and enclosed when the weather demands it. The Dallas Cowboys stadium, currently under construction, has a budget of one billion dollars (Fig. 1). Stadium design and construction creates special demands. Among them are vibration control, durability and fast construction speed. These requirements are a prefect fit for the application of precast prestressed concrete. Pretensioning of seat sections provides a stiff, crack free member. Speed can be achieved because construction can be telescoped with production in precast plant proceeding while job site foundations advance. In addition high strength precast members are more durable, and the tolerance control of precast members produces fast erection and stadium completion (Fig. 2). The most typical precast member to all stadiums is the seat riser section. They have been cast one riser, two riser and three risers high and are pretensioned. The most typical shape is two risers high with lengths up to 42 feet. Vomitory members are also typically precast, as are wall panels and stairs. Precast concrete has been also employed in raker beams, columns, ramps and concourse framing.the fixed roof or retractable roofs have all been steel construction except for fabric roofs. Figure 3 is a section through a typical precast stadium framing plan. As noted previously, precast seat members (risers) are the most common precast product in stadium construction. They are typically cast two levels high (each Figure 1. Dallas Cowboys Stadium. Figure

Figure 3. Section of a typical concourse and seating area framing. Figure 5. Match cast moment frame. Figures 4a & 4b. Section of a typical stadium riser.

2 Figure 3. Section of a typical concourse and seating area framing. Figure 5. Match cast moment frame. Figures 4a & 4b. Section of a typical stadium riser. 1 8 high) and the tread is 2 8 wide and are pretensioned with span up to 42 feet.the key design feature is to minimize excessive deflection and vibration in order to not cause the occupants concern. Excessive vibration can lead to panic in the stands. The live loading in a stadium is very dynamic and must be accounted for. Most precast prestressed standard sections are symmetrical about their vertical axis. However, stadium riser units are not (Fig. 4) and it is necessary to calculate properties about the principal axis. Strength design is complex (Fig. 4b) and may be approximated or computed graphically using automated computer programs. Regarding vibration control, the seating units have a three-dimensional nature and vibrate and deflect about their weakest principal axis. The minimal natural frequency of the cross-section is first determined which is then compared to the forcing frequencies in Hertz. Excessive deflection in supporting beams such as rakers must also be controlled to make sure the occupants aren t alarmed. While ideal sight lines would require many variable riser heights for economical reasons, primarily form costs, it is best to limit the numbers of different riser heights. Raker beams with their saw toothed tops are the key member supporting the seating units (Fig. 3). The top most raker beam is typically a long member with a simple span and a cantilever at each end. Because of the heavy dynamic load these members typically have a large cross-section. They are typically cast on their side and are pretensioned for handling and erection. Once in place and under load they are post-tension to provide the remainder of the load capacity primarily the negative moment at the cantilever. These raker beams can be located very high in the air; therefore, casting-them-place is difficult. In addition the tolerance control employing precast raker beams is much superior over cast-in-place beams. Cast-in-place raker beams frequently require a great deal of shimming and product trimming to get the seat section to properly fit. While precast raker beams have occasionally been supported by cast-in-place columns, the more typical and efficient method is to employ precast columns (Fig. 5). Since the columns are typically over 100 feet long they are broken into several pieces and spliced with various means. These tall columns also can support intermediate concourse levels or lower raker beams. These tall columns and horizontal beams at different levels were connected by vertical post-tensioning to create moment frames to provide 864

Figure 6. Sun Coast Dome. Figure 7. A schematic of stadium showing the basic roof framing system. a very stable base for the concourse and upper raker beam (Fig. 5).

Although an elaborate forming system was required for the match casting, the erector was able to proceed at a fast rate and no expensive scaffolding was required to grout the joints.

3 Figure 6. Sun Coast Dome. Figure 7. A schematic of stadium showing the basic roof framing system. a very stable base for the concourse and upper raker beam (Fig. 5). The beams and columns were match cast at each level so that no grouting and dry packing of the joints was required. Although an elaborate forming system was required for the match casting, the erector was able to proceed at a fast rate and no expensive scaffolding was required to grout the joints. They were sealed with epoxy applied on the ground just prior to lifting. These moment frames for the Citrus Bowl Stadium in Orlando, Florida were designed for hurricane intensity winds and lateral loads. Similar precast frames have been designed and constructed to resist seismic loads. I shall now describe three entirely different projects that employed total precast solutions for the structural framing. The original design of the structure called for a cast-in-place concrete framing system which was over budget with a poor construction schedule. The Consulting Engineers Group working with a precaster offered a total precast concrete design alternate which provided savings in cost and time. The Consulting Engineers Group was then hired to completely redesign the cast-in-place frame to a precast system. The stadium construction consists of two basic components, each made up of several unique precast members. The first component is the ring beam which supports the cable roof system. The second component is the concourse and seating area framing. The ring beam system is comprised of 6 diameter hollow column, 170 long cast in 56 lengths. Sitting atop the column is a steel knuckle assembly which supports the twin ring beam members each 8 6 high, 16 thick and 90 long (Fig. 8). A precast deck slab span between the two parallel ring beams completing the ring framing system. The seating and concourse framing was more or less conventional stadium framing with precast pretension seat sections, combination pretension and post-tensioned raker beams, precast columns and beams, vomitory panels and slabs, and concourse double tees plus precast circular pedestrian ramps. The floor member of the circular ramp was a special double tee cast in warped position to accommodate the varying inside and outside slopes. Special site erection requirements necessitated the use of a precast dead men which were made up of precast blocks that were bolted together then as needed at another location disassembled and rebolted together. 1 SUN COAST DOME, ST. PETERSBURG, FL While domed stadiums built prior to the Sun Coast dome had employed various precast components, the Sun Coast Dome was the first domed structure in which all structural elements and elevated seating are completely precast concrete (except for the cable supported fabric roof). (Fig. 6) The diameter of the stadium is 688 (210 m) with a height of 225 (69 m). The cable supported roof employs a tension cable and compression post system originated by Bucksminster Fuller termed a tensegrity system which produces an exterior compression ring and an interior tension ring (Fig. 7). 865

Figure 9. Completed Citrus Bowl. Figure 8. Cross-section of assembled ring beam showing post-tensioning and mild steel reinforcement.

2 CITRUS BOWL STADIUM, ORLANDO, FL This is another all precast structure system which was bid as a design build project.

Vibration control was of extreme importance since a previous steel frame stadium was turned down because of excessive vibration causing panic in the stands the precast solution performed extremely

4 Figure 9. Completed Citrus Bowl. Figure 8. Cross-section of assembled ring beam showing post-tensioning and mild steel reinforcement. The stadium, containing over 7,000 precast components, was constructed on time and within budget representing considerable savings to the owners. Figure CITRUS BOWL STADIUM, ORLANDO, FL This is another all precast structure system which was bid as a design build project. Besides the tall, spectacular precast framing system, the structure was unique in that the penalty damages if one key completion day was missed were $24,000,000 for the one loss day. Vibration control was of extreme importance since a previous steel frame stadium was turned down because of excessive vibration causing panic in the stands the precast solution performed extremely well in vibration control. Several design and construction decisions were made to speed construction and make sure the deadline was met. The upper seating section is over 170 from the ground and the available footprint only allowed a 28 column spacing for the building frame. To withstand hurricane wind forces, the frame components were vertically post-tensioned to the foundation using post-tension bars. The raker beams were also connected to the support columns by post-tensioning. To speed up erection, it was decided to match cast the support columns to the beams located at various concourse levels. This process eliminated the need for Figure 11. University of Oregon Stadium. grouting as dry packing from scaffolding of frame joints (Fig. 5). Precast pretension double riser tee seat sections were also employed. In addition, to complete the all precast solution, the sloping ramps were constructed with load bearing precast architectural finish panels and warped precast floor slabs to create a constant sloping ramp. The total precast framing system met the tight one-year construction schedule (from Bowl game to Bowl game) and was completed one month before the deadline (Fig. 9). 866

5 3 UNIVERSITY OF OREGON STADIUM, EUGENE, OR This stadium was also conceived to be constructed with a cast-in-place framing system. However, cost over runs and scheduling problems lead to the consideration of a total precast redesign. The redesign was conducted by The Consulting Engineers Group and accepted by the architect. The architectural design of the primary bent supports for the raker beams were large V shaped frames. As drawn, the frames were too large to precast and ship. The legs of the V were then split into two components and match cast. The V components were reassembled in the field. The joints epoxy sealed and the section post-tensioned together (Fig. 10). The stadium is located in the west coast, high seismic region so all precast components and their connections required consideration of seismic forces. This stadium was constructed to meet a tight schedule related to the University football schedule (Fig. 11). 4 CONCLUSION Precast framing solutions for major sport stadiums has proven to be a viable economical solution and able to meet extremely tight schedules. Careful design considerations of vibration characteristics of precast seating members have produced structures that are vibration stable providing security and comfort to attendees. 867

PRECAST CONCRETE STRUCTURES

PRECAST CONCRETE STRUCTURES 1. INTRODUCTION The concept of precast (also known as prefabricated ) construction includes those buildings, where the majority of structural components are standardized and